Gait mechanics of inverted walking: Implications for the evolution of suspensory locomotion

A shift from above-branch quadrupedalism to suspensory locomotion is viewed as a critical transition in primate locomotor evolution, particularly as larger primates moved to suspensory postures as a means to mitigate the problems associated with balance on relatively thin arboreal supports. While such a strategy reduces problems associated with balance, it may change the mechanical stresses on the limbs and obviate energy saving mechanisms used during above-branch quadrupedalism. Currently, little is known about the basic mechanics and requirements of this unusual form of locomotion and the changes in anatomy required to successfully adopt effective inverted quadrupedalism. This study examined the mechanics of inverted quadrupedalism in three adult Varecia variegata walking above and below an instrumented raised pole. During upside-down quadrupedalism animals had a significantly shorter stride duration and swing phase, and a longer stride frequency and forelimb duty factor compared to above-branch walking. Kinetic analyses shows that during inverted walking compared to above-branch walking the normal pattern of peak vertical forces was reversed (FL/HL Vpk ratio = 0.730 vs. 1.338) and the braking and propulsive roles of the forelimb and hindlimb are reversed (FL is net propulsive). The shift between above-branch to below-branch quadrupedalism appears to involve significant alterations to the rate, direction, and magnitude at which the limbs are loaded that reflect a change in the functional role of the forelimb and hindlimb. Habitual inverted quadrupedalism in certain primate lineages may have played a key role in the reduction of hindlimb loading and the evolution of suspensory locomotion.

This project was funded by the National Science Foundation's Graduate Research Fellowship Program (GRFP)